Apparatus for molding and firing dental porcelain

ABSTRACT

Method and apparatus whereby powdered porcelain or plastic material is charged in the opposing cavities of half flasks, which cavities have been formed about a pattern within the investment of each flask half and wherein the flasks are placed in separated condition within the muffle of a furnace and such separation is maintained until furnace heat is achieved slightly in excess of the liquefying temperature of the powder in the flask and then said flask halves are joined to compress the charge to fill all parts of the molded cavity. Refractory spacer means sensitive to temperature maintain separation between the flasks until the critical temperature is reached. The flask halves may thereafter be compressed together either manually or mechanically. The apparatus includes a tiltable furnace and visual observation of the spacer from the furnace exterior.

United States Patent MacDonald, deceased et al.

[54] APPARATUS FOR MOLDING AND FIRING DENTAL PORCELAIN [72] Inventors:Ronald F. MacDonald, deceased, late of Orange, Calif; by Albert J.

MacDonald, executor, Van Nuys, Calif.

[73] Assignee: Joseph L. MacDonald, 'Medfield,

Mass.

22 Filed: Sept. 8, 1970 21 App1.No.: 70,154

[521 US. Cl. ..425/l7l, 425/174 I-IR, 425/405, 425/408 [51] Int. Cl..B29c 3/00 [58] Field of Search ..18/5.7, 17 D, 47 D, 34.1, 33, 18/32,DIG. 60, DIG. 62, 17 R, DIG. 12;

[ 51 Sept. 26, 1972 3,200,454 8/ 1965 Gramenzi 164/378 X 2,069,7462/1937 Andrews ..18/33 X 3,411,184 11/1968 McGowan ..18/33 3,109,9111l/1963 Kremer ..l3/31 X Primary Examiner-4. Spencer OverholserAssistant ExaminerDavid S. Safran Attorney-Wm. Jacquet Gribble 5 7]ABSTRACT Method and apparatus whereby powdered porcelain or plasticmaterial is charged in the opposing cavities of half flasks, whichcavities have been formed about a pattern within the investment of eachflask half and wherein the flasks are placed in separated conditionwithin the muffle of a furnace and such separation is maintained untilfurnace heat is achieved slightly in excess of the liquefyingtemperature of the powder in the flask and then said flask halves arejoined to com press the charge to fill all parts of the molded cavity.Refractory spacer means sensitive to temperature maintain separationbetween the flasks until the critical temperature is reached. The flaskhalves may thereafter be compressed together either manually ormechanically. The apparatus includes a tiltable furnace and visualobservation of the spacer from the furnace exterior,

9 Claims, 9 Drawing Figures PATENTEDsEP2s I972 SHEET 1 BF 3 INVENTORRONALD F. MccDONALD T TOP/VET PATENTEDSEPZE I912 3.694.122

SHEET 2 OF 3 FIG. 3. 64

49 INVENTOR RONALD F. MOCDONALD T TORNE Y PATENTEflsiPzs I972 SHEET 3 BF3 INVENTOR RONALD F. Mac DONALD F/QQ.

T TORNE V APPARATUS FOR MOLDING AND FIRING DENTAL PORCELAIN BACKGROUNDOF THE INVENTION The invention relates to method and apparatus forprocessing fired dentures and more particularly to method and apparatuswhereby porcelain or plastic powder, which may be tinted, is molded andfired to create quality artificial denture components, such as porcelainmountable on gold, porcelain .bridgework and inlays.

Naturally tinted or colored porcelain or plastic teeth have beenpreviously fabricated by highly skilled dental technicians. Theconventional process is laborious and not dependable, relying on theartistry and training of the artisan. Quality denture replacement partsare generally fired porcelain on a gold base. The porcelain must be of aparticular quality in order to have the same coefficient of expansionand contraction as the gold and to adhere to the gold, and in order toaccept colors which are natural appearing. Gradation of color from onepart of the artificial tooth to the other is very important. The methodand apparatus of the invention achieve natural-appearing dentalprosthetic materials by a method and apparatus which may be implementedby semiskilled craftsmen in a manner which does not require the longexperience and keen perceptions of a highly trained craftsman.

While porcelain is used to describe the substances ,of which the dentalmaterials are made, it is to be understood that the term as used coversall of those porcelain-like substances capable of ceramic response toheating and cooling. The substances for dental materials such asbridges, plates, etc. must not only survive a range of temperatures, butmust also result in artificial teeth which look natural, remainunchanged after years of exposure to mouth chemicals and foodstuffs. Tolook natural the material of the teeth must accept coloring agents whichcan remain gradated through the firing step, dispersed in the materialin natural-appearing color differentiation from one area to another.

SUMMARY OF THE INVENTION The process of the invention contemplates thesteps of packing an anterior flask half around a conventionally preparedwax pattern with an investment in a plastic state, which hardens aboutthe pattern. The posterior flask half is then packed in like manner witha plastic investment and the flask halves are joined so that each halfreceives the imprint of the pattern. The flask halves are then separatedand the pattern is melted from the flask halves. The anterior flask halfcavity is then packed with powdered porcelain previously blended withproper coloring agents, while joined to the posterior half so that thecavity is filled. A parting coating on the investment surface betweenthe flask halves aids separation after they are joined. An overcharge ofporcelain powder is then introduced through a vent hole into the flaskand the flask is vibrated. The flask halves are then separated slightlyand a ceramic spacer rod is placed between the flask halves. The joinedflask halves are then mounted on an articulated portion of the furnaceso that the flask extends into the muffle of the furnace, posterior halffirst. The furnace is then fired and the temperature brought to themelting point of the porcelain powder and the spacer rod. The spacer rodmelts, indicating critical temperature and the flask halves are joined,compressing the liquefied porcelain powder within the cavity formedabout the lost wax pattern.

The invention contemplates unique apparatus for implementation of theinventive method. A vacuum furnace, preferably electric, has a housing,a door on the housing and a muffle, or heating unit within the housing.The housing is pivotally mounted upon a control console. A support beamslidable in the housing door supports removably a separable mold flaskwhich, when closed, supports the flask within the muffle, The flaskcontains an investment surrounding the mold cavity. The cavity connectswith waste vents extending exteriorly of the muffle. The support beambears against the anterior flask half. A stop pin bears against theposterior flask half. The beam is spring loaded to thrust the halvestogether, with a spacer rod of ceramic material separating the halvesuntil the rod melts.

In a preferred embodiment of the invention the muffle is movably mountedwithin the furnace housing by radial thrust springs anchored inlongitudinal bearing bar pairs spaced circumferentially about themuffle. Preferably both the muffle and the housing are apertured toprovide an observation port through which the spacer rod of the moldflask may be inspected in place.

The inventive furnace includes aflask which is reusable, the flaskcomprising outer shell halves each of which is impervious to the furnaceheat and a hardenable pourable investment which deteriorates in thefurnace heat so as to be removable from the shell.

The inventive process and apparatus work together to produce prostheticdental materials of superbly natural appearance which can be used in anydental restoration device because of their beauty, durability andversatility.

These and other advantages of the invention are apparent from thefollowing detailed description and drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of apreferred embodiment of the apparatus invention;

FIG. 2 is a longitudinal section of the embodiment of FIG. 1;

FIG. 3 is a section similar to FIG. 2 with the mold flask in the furnacemuffle;

FIG. 4 is a sectional elevation taken along line 4-4 of FIG. 3;

FIG. 5 is a fragmentary sectional view of a mold flask on the supportbeam of the housing door;

FIG. 6 is a view taken along line 6-6 of FIG. 5;

FIG. 7 is a sectional view taken along line 77 of FIG. 5;

FIG. 8 is a fragmentary sectional view along the axis of the closedfurnace showing the flask halves joined; and

FIG. 9 is a side elevation of the furnace of the invention in operatingposition.

DESCRIPTION OF THE PREFERRED EMBODIMENT An understanding of the entireprocess the ceramist performs to achieve dental prosthetic materials ishelpful in the comprehension of the invention. The general and aninvestment is packed about the pattern within the flask half. Theinvestment hardens about the pattern and the exposed face of theinvestment is coated with a lubricant separator, such as Vaselinef. Theposterior flask half is also. packed with investment and the flaskhalves joined such that each half is impressed by the wax pattern. Theflask halves are then separated and the wax pattern is melted fromtheflask half. Depending on the type of vpattern material used, the waxmaybe lost from the surrounding investment by immersion in boiling wateror may be burned out at a temperature of about l,500 F.

The pattern cavity in the flask half is then filled with powderedporcelain which has been blended with color agents. Preferably theporcelain charge is of a type .with a high feldspar content which isself-glazing and contains uranium and nickel cobalt oxides. Coloringagents are preferably mixed with the powdered porcelain prior to itsplacement in the moldjflask. Depending upon the dental device desired,the charge may be composed of varying amounts of these components so asto'melt at a chosen point between 1,600" and 1,850 F A lubricatingseparation layer intervenes between flask halves before the halves arejoined. The charged flask halves, after joining, are overcharged throughthe ventholeof the flask while the flask is vibrated. The .vibrationtends to solidify the powder so that it remains within the flask whenthe halves are separated. Preferably the overcharge is equal to 5 to 20percent of the capacity of the pattern cavity in terms of normal powderdensity. After the charge is solidified the flask halves are separatedand a refractory rod is placed between the mating faces of the flaskhalves. The halves are then placed on the support beam of the furnacedoor and the door is closed to position the mold flask within the muffleof the furnace. The charged flask is then subjected to the furnace heatunder vacuum, while the flask halves, under joining tension, areseparated by the spacer rod.

I The spacer rod is of a refractory or ceramic material compounded suchthat its melting point is no less than the melting point of theporcelain-charge in the flask. Preferably the rod melts atapproximately50 higher temperature than the charge, insuring that thecharge has indeed reached its critical fluid condition.

As soon as the melting point of the charge and the rod is reached thetension applied to the flask halves joins them. The charge is thuspressed in the cavity by the joining action of the flask halves at theproper temperature to achieve quality ceramic material. 7 The flask isthen removed from the furnace, the mold flask halves separated and theinvestment removed from about the press-molded porcelain, which can thenbe subjected to further steps in completing the molded and firedprosthetic device.

Since the porcelain powder for the charge, and the spacer rod, can bemanufactured to very close tolerances in terms of melting point andsince the desired temperature at which the porcelain charge should bepressed is known, the process achieves exact timing to result in qualityporcelain, without the necessity of highly trained ceramists toimplement the process.

Further details of the process are set forth in connection with thedescription of the operation of the apparatusof the invention whichfollows:

v The furnace, 11 of FIGS. 1-9 may be controlled from a conventionalconsole 12 which has a conventional heat dial 13, heat indicator andthermostat control 14,

vacuum gauge 15 and control switch bank 18. A switch 19 controls thevacuum pump (not shown).

A vacuum hose 21 (FIG. 3) proceeds within a flexible hose conduit 22from the pump to a cylindrical housing 23 of the furnace 1 l. Thehousing is suspended above the console on uprights 25, 26 attached togimbels like gimbel 27 which have tension nuts 28 so the attitude of thefurnace with respect to horizontal may be controlled.

Visual inspection of the interior of the furnace is desirable andtherefore the housing is provided with an observation port indicatedgenerally at 31. As can be seen from FIGS. 2 and 3, the port comprises aconventional lens casing 33 within which a quartz lens (not shown) issealed to close the furnace to both heat and vacuum loss. The casingseats within a stepped aperture 34 of the housing wall 35. A muffle 36in the housing has an exterior wall 38 and an interior wall 39. The port31 is in line with openings 41, 42 ofthe exterior wall 38 and interiorwall 39, respectively. The furnace housing 23 comprises the cylindricalwall 35, a back wall 44 and a rod hinged-mounted front door 45. Thecylindrical wall 35 is pierced opposite port 31 to receive a sealedvacuum hose coupling 46 to which the inner vacuum hose 21 iscoupled.Flexible shroud 21 also contains electrical leads 47, 48, 49 whichextend from the controls of the console 12 to the muffle 36. Whilevarious types of heating elements may be used to practice the invention,the preferred muffle is one like that described in copendingapplication, Ser. No. 771,409 for Muffle Furnace filed Oct. 29, 1968,now US Pat. No. 3,541,293.

The muffle has front and rear refractory walls 52, 53 in addition to thecylindrical outer case wall 38 and the inner spacer wall 39 betweenwhich a plurality of circumferentially spaced heating coils 56 extendwithin tubes 57 whose nature is best comprehended with respect to theabove mentioned copending application. Wall 52 has an opening 59affording access to the muffle interior. The muffle is removablypositioned within the housing by means of a plurality of pairs of spacedbearing bars 61, 62. The bars of each pair are thrust apart by a pair ofradial compression springs 63, 64, visible in FIGS. 2 and 3.

As can be seen with respect to FIG. 4, the bearing barpairs are spacedperipherally about the muffle such that the thrust upon the muffle byone pair is counterbalanced by a diametrically opposite bearing barpair. The compression springs hold the muffle firmly in the desiredlocation and the muffle may be easily removed to be repaired or replacedby overcoming the spring pressure. A stop rod 71 extends through theback wall 53 of the muffle from an adjustment sleeve 72 which intervenesbetween wall 53 and housing wall 44. Stop 71 is movable within sleeve 72through the back wall and is fixed in desired position by a set screw74. The stop is used to locate a divisible mold flask 80 with itsposterior flask half 81 and anterior half 83 with respect to the muffleand more particularly with respect to observation port 31 and theopenings 34, 41, 42 in the housing and the muffle walls. The sleeveregisters the mufile in the housing so that the muffle openings alignwith port 31.

The anterior flask half 83 has an extending hollow boss 84 into which acylindrical support beam 85 fits. The beam passes through and holds amuffle closure 88 and also passes through a threaded gland 89 which issealed within a threaded aperture'90 of furnace door 45. a

An exterior thread 91 of the gland receives a tension nut 92 withinwhose cavity 93 a compression spring 94 and a thrust collar 95 reside.The thrust collar bears against a shoulder 96 formed on the beam by thereduction of beam 85 to a small stem 97, which projects outwardly fromthe outer wall of nut 92. (See FIG. 8) An O-ring 98 seals between thebeam and gland 89. A larger compression spring 99 surrounds the beam 85,thrusting muffle closure88 away from the interior surface of door 45. Ashield 101 below the opening 59 of the muffle protects the electricalleads to the muffle heating elements and a lead 105 to a heat sensor106. The shield 101 also aids in guiding closure 88 into the muffleopening 59. All of the electrical leads extend from the muffle throughthe housing wall 35 into the. flexible conduit 22 and thence to thecomponents of the control console. Conventional heat resistant grommetslike the grommet 108 seal the wall passages'of the leads to the conduit.

In FIG. 2 the door 45 of the furnace is shown in open position,supporting mold flask 80 on beam 85 exteriorly of the furnace, with theflask halves 81, 83 separated by a ceramic spacer rod 111. In FIG. 3 thedoor is closed and the mold flask is positioned against stop 71 withinthe furnace muffle such that spacer rod 11 1 holds the halves apartalong a line coinciding with observation port 31 and the openings 41, 42in the muffle. The rod may be of glass, porcelain, a blend of both orother fritted materials.

The door is suspended from a pair of diametrically spaced articulatedarms like arm 121 of FIG. 1. Each Each arm is fixed in a radial ear 123of the door and journalled in a radial tab 125 of the housing. Each armhas a pivot joint 126 of conventional nature spaced an interval from armhead 127 which acts as a stop against extending each arm beyond thehousing tabs.

When the door 45 is in the position of FIG. 2 each arm 121 extendsforwardly of housing tabs 125 such that the articulated joint 126 isoutward of the tab 125. Thus the door may be spaced outwardly from thefurnace along the central axis of the furnace housing and then bentdownwardly, as shown in FIG. 2, such that the muffle closure 88 and themold flask may be removed from the muffle through opening 59 and thenthe door tilted downwardly about the arm joints 126. Conversely, whenthe flask is to be loaded into the furnace, the door 45 may be placed inthe attitude of FIG.

2 with the upper end of beam 85 ready to receive the flask, with themuffle closure 88 held away from the door by the spring 99. Once theflask is in position on the support beam the door may be swung upwardlyuntil joints 126 are aligned with arms 121 and then the door may beclosed axially inwardly, introducing the flask into the mufi'le andbringing the closure into the opening 59. The vacuum induced in thefurnace holds the door closed. Conventional sealing methods may be usedto make the closure tight, such as O-ring 131 in a groove 132 of theinner door face and conventional door latches (not shown) may be used toinsure door closure.

Turning now to FIGS. 5-8, which illustrate the preferred mold flask ofthe invention, the flask is shown in FIGS. 5 and 7 in the position itoccupies within the muffle when first introduced into the furnace.

In FIGS. 5 and 7 the posterior flask half 81 is shown to have a partialrear wall 133 which registers against stop 71 when introduced into themuffle. The posterior half has a cylindrical side wall 134 withdiametrically opposed locator tabs 135, 136 extending oppositely fromthe wall 133. Ninety degrees circumferentially from each tab 135, 136 isan internally extending boss 137, 138 with a guide aperture 139 therein.Each boss is notched at 141. The flask half, including the notches 141,is filled with an investment 142, now hardened.

The anterior flask half 83 has an anterior wall 145 with an anteriorcentral boss 84 having a stepped recess 146 in which an inner end 147 ofthe support beam lodges.

A cylindrical cup 149 separable from wall has an outer ring wall 151with diametrically opposed spools 152 which receive guide pins 153, 154.The pins are fixed in wall 145 of the anterior flask half and extendposteriorly beyond the anterior half to register in the bosses 137, 138of the posterior half.

The flask anterior half defined by wall 145 and cup 149 contains aninvestment 156 which fills the mold flask half about the spools 152 andabout a wax-formed cavity 159 now filled with powdered porcelain 161within the cavity. The porcelain is introduced into the investmentcavity 159 through a previously formed conical opening 162 and a spruegate 163. The gate connects with one of a plurality of openings 164diametrically aligned in wall 145 of the anterior flask half. Since thevarious patterns form differently placed cavities in the flasks, one ormore of these openings may connect with various sprue gates, dependingupon the position within the investment and the attitude of the waxpattern previously therein.

When the investment was shaped, the conical cavity 162 was filled by aconical central protrusion 167 of an investment 171 of the posteriorflask half. When the two'halves were united about the wax pattern (whichfilled the space now occupied by powdered porcelain charge 161) apatterned concave recess 173 resulted in the investment 171 of flaskhalf 81.

In FIGS. 5, 6 and 7 the halves are separated by refractory or ceramicspacer 111. The spacer ends reside in cavities cored in the investmentat the time the investment is poured about the wax pattern. Alignment ofthe spacer rod and the pattern cavities is maintained by the guide pins153, 154 and by the peripheral tabs opposed pairs 176,

.their decomposition under heat and the difficulty of removing theinvestment. However, the flask of the invention comprises a shellcombined with an investment which deteriorates under the process heatsuch that it is easily removed. The chrome cobalt flask is filled withan investment which, when mixed to be plastic, is composed of liquidphosphoric acid, ground magnesium oxide and Portland cement. By bulk theproportions of the three principal ingredients are phosphoric acid 1,magnesium oxide 1 and Portland cement 0.5, depending on concentrationand fineness.

' This particular investment is degraded by the furnace heat such thatit is easily removed from the unaltered flask shell, making the flaskshell reusable and reducing the unit cost of ceramic materials made withthe process and furnace of the invention. The above described furnace isused to implement the process in the following way:

The flask halves, prepared as previously described, are charged andovercharged with porcelain powder of the high feldspar type.Conventional separators are placed on the facing surfaces of the flaskhalves. The anterior flask half is placed upon the support beam 85 ofthe open furnace door while the door 45 is in the attitude of FIG. 2. Aspacer rod, such as the rod 111, which is precisely compounded so as tomelt at approximately 50 higher than the melting point of the charge, isplaced between the anterior flask and the posterior flask. The posteriorflask is located upon guide pins 153, 154 as close to the anteriorflaskhalf as the-spacer rod permits. The door is then tilted upwardly fromthe position shown in FIG. 2 and the arms 121 of the door 45 pushedthrough the tabs 125 of the housing such that the support beam carriesthe mold flask through opening 59 of the muffle until it registersagainst stop 71. During this maneuver closure 88 of the muffle isimpelled into the opening under the thrust of compression spring99.'Tension nut 92 is then adjusted such that compression spring 94thrusts support beam 85 against anterior flask half 83 sufficiently tojoin the flask halves when spacer rod 1 l 1 melts. The spring load onthe support beam tending to join the halves is about 30 pounds. Thecontrols of the console are activated such that the requisite heat isreached and the furnace interior is exhausted to the proper vacuumpoint. The normal vacuum operatingrange is from 25 to 29 inches ofvacuum. Conventionally firing of dental porcelain is done between l,600to l,850 F.

As soon as the flask has entered the muffle the furnace housing mamatilted about girnbels 28 from the broken line position 180 of FIG. 9into the solid line position shown in FIG. 9. This inclined position notonly insures against the accidental displacement of the charge from theflask,.but also brings observation port 31 into convenient position forthe furnace operator to observe the condition of the mold flask withinthe furnace and particularly to inspect the spacer rod.

Upon observing the melting of the spacer rod and the spring-inducedclosure of the flask halves, the tension nut 92 may be turned toincrease the torque on the anterior flasks to insure perfect closurebetween the two flask halves, compressing the charge in the cavity.Alternatively, the operator may choose to observe the melted conditionof the spacer rod and apply force to the support beam by adjustment ofnut 92 at that time instead of adjusting the out prior to the firing ofthe furnace.

After the furnace has cooled ambient air is restored and the door may beopened in the manner previously described after the furnace is returnedto the generally horizontal position of FIG. 1. The flask may then beremoved, cooled and the press-molded ceramic piece removed from themold. The finishing procedure for most dental prosthetics is thenfollowed, such as staining, additions and glazing.

While the support beam is an element of the embodiment of the furnace asexplained above, it may be removed should a different type of flask bedesired. Conventional furnace operating techniques may be used with thefurnace with or without the support beam. The altered furnace thereforehas utility as a conventional furnace as well as being operative toimplement the invention process.

Variations in the physical arrangement of the furnace and the supportbeam will occur to those skilled in the art. The disclosed embodimentsare illustrative only and it is desired that the scope of the inventionbe measured by the appended claims.

What is claimed is:

1. An electrical vacuum furnace having temperature and vacuum controlsand comprising a housing, a door sealing the housing, a mold flaskadapted to hold a charge, said flask having seperable halves, bias meansfor urging the separable flask halves, together, destructable spacermeans for separating the halves of the flask against the-urging of thebias means during initial heating, said spacer means having a meltingpoint no lower than the melting point of the charge in the flask, stopmeans for the flask in the housing, means to heat the contents of thefurnace to a molding temperature higher than the melting point of saidspacer means thereby destroying said spacer means and causing said flaskhalves to close, and-means for suspending said flask proximate theheating means.

2. Apparatus in accordance with claim 1 wherein the spacer meanscomprises a ceramic rod, said ceramic being chosen from a classconsisting of porcelain, plastic, glass and a fritted combination.

3. Apparatus in accordance with claim 1 wherein the flask support meanscomprises a beam slidably mounted in the door.

4. Apparatus in accordance with claim 1 .wherein the means urging theflask halves comprises an adjustable compression spring bearing againstthe flask half and the housing door.

5. Apparatus in accordance with claim 1 wherein the means urging theflask halves together comprises a beam joumalled' in the housing door,said beam bearing against the flask, a thrust shoulder on the beamexterior of the door and a threadably adjustable collar movable withrespect to the door to bear against the shoulder and thrust the beamagainst the flask to alter its spacing with respect to the second flaskhalf.

6. Apparatus in accordance with claim 1 wherein the fumacefurthercomprises an observation port in the housing and means for positioningthe flask within the housing such that the space between flask halves isoptically aligned with the observation port.

7. Apparatus in accordance with claim 1 further comprising swivel meansmounting the furnace housing to pivot about a horizontal axisperpendicular to the joining motion of the flask halves. v

8. Apparatus in accordance with claim 1 wherein the heating elementcomprises a substantially cylindrical muffle, an observation paththrough the mufile perpendicular to its axis, means for adjustablymounting the mufile within the housing and an adjustable stop rodextending into the mufile to limit motion of the posterior half of theflask.

9. Apparatus in accordance with claim 8 wherein the means mounting themufile comprises a plurality of bearing bar pairs, each bar pair beingspaced from an adjacent bar pair about the exterior of the muffle andcompression springs separating the bars of each pair such that one barbears against the interior of the furnace housing and the other bar ofthe pair bears against the exterior of the cylindrical muffle.

2. Apparatus in accordance with claim 1 wherein the spacer meanscomprises a ceramic rod, said ceramic being chosen from a classconsisting of porcelain, plastic, glass and a fritted combination. 3.Apparatus in accordance with claim 1 wherein the flask support meanscomprises a beam slidably mounted in the door.
 4. Apparatus inaccordance with claim 1 wherein the means urging the flask halvescomprises an adjustable compression spring bearing against the flaskhalf and the housing door.
 5. Apparatus in accordance with claim 1wherein the means urging the flask halves together comprises a beamjournalled in the housing door, said beam bearing against the flask, athrust shoulder on the beam exterior of the door and a threadablyadjustable collar movable with respect to the door to bear against theshoulder and thrust the beam against the flask to alter its spacing withrespect to the second flask half.
 6. Apparatus in accordance with claim1 wherein the furnace further comprises an observation port in thehousing and means for positioning the flask within the housing such thatthe space between flask halves is optically aligned with the observationport.
 7. Apparatus in accordance with claim 1 further comprising swivelmeans mounting the furnace housing to pivot about a horizontal axisperpendicular to the joining motion of the flask halves.
 8. Apparatus inaccordance with claim 1 wherein the heating element comprises asubstantially cylindrical muffle, an observation path through the muffleperpendicular to its axis, means for adjustably mounting the mufflewithin the housing and an adjustable stop rod extending into the muffleto limit motion of the posterior half of the flask.
 9. Apparatus inaccordance with claim 8 wherein the means mounting the muffle comprisesa plurality of bearing bar pairs, each bar pair being spaced from anadjacent bar pair about the exterior of the muffle and compressionsprings separating the bars of each pair such that one bar bears againstthe interior of the furnace housing and the other bar of the pair bearsagainst the exterior of the cylindrical muffle.